ScratchJr (iOS only)
Scratch but designed for smaller kids (ages 5-7 according to their own site). Currently only iOS version, Android version coming later.

Hopscotch (iOS only)
Another graphical programming environment designed for kids. Has some game-like "episodes" where you have to perform certain task or just create your own programs/games like with Scratch.

App Inventor
Graphical programming environment to create Android apps. Quite similar to Scratch, but not as easy to use.

Pocket Code
Another graphical programming environment for Android that is similar to Scratch. You can publish programs and download and remix other people's programs and even convert them to standalone Android apps. It also has (an experimental) feature to import Scratch projects into it, so it's an easy way to convert your existing Scratch programs to mobile apps.